1. Field of the Invention
The present invention relates to a color display tube for high resolution and, more particularly, to a color cathode ray tube with an in-line electron gun.
2. Description of the Related Art
The description will be made first on a conventional color cathode ray tube with reference to FIG. 4. In this drawing, three electron beams 11R, 11G, and 11B generated by an in-line electron gun 11 are used for red, green and blue fluorescent materials, respectively, and pass through a hole 12a of a shadow mask 12. On the other hand, there are provided stripe-shaped fluorescent materials 13R, 13G, and 13B of red, green and blue on a display screen 13 of a glass bulb or tube (not shown). Accordingly, the central electron beam 11G allows the green fluorescent material 13G to shine, whereas outer electron beams 11R and 11B allow the red and blue fluorescent materials 13R and 13B to shine, respectively.
Referring to FIG. 5 the electron gun 11 (FIG. 4) includes an electrode 14 in the main lens thereof. There are provided three apertures 14R, 14G, and 14B in the main lens of the gun 11. These apertures 14R, 14G and 14B are designed to be the same in diameter as one another for such reasons as to prevent imbalance of the strengths of electrostatic lenses produced at each electrode and to hold the strength of the inner electrode 14.
In order to enhance the focus property of the color cathode ray tube, it is required to improve the spherical aberration of the electrostatic lens formed among the electrodes. In general, in order to provide an electrostatic lens small in the spherical aberration, it is effective to enlarge the aperture diameter of the main lens unit.
However, it is impossible for the circular aperture-type electron gun 11 shown in FIG. 5 to enlarge the diameter of each aperture 14 due to the strength of the inner electrode 14.
Moreover, although it is required to correct the convergence for focus the three electron beams 11R, 11G and 11B on the screen surface, such correction can be done with the help of the ferrite and/or the magnet or other related materials with respect to the red electron beam 11R and the blue electron beam 11B. However, it is impossible to correct the green electron beam 11G with such means.
Furthermore, in the circular aperture-type electron gun 11, the central electron gun is inferior, in the electron gun's characteristics, to the outer electron guns. This emanates from the experimental comparison, made at the center of the screen, in terms of their focus property.
In addition to that, the central electron beam corresponds to green, and its electric current is more than those of the outer electron guns, in operation. This leads to further degradation of the focus property.
Furthermore, the brightness characteristic is the one to influence the outward appearance on the screen of the cathode ray tube.
The easiest method to enhance the brightness is to enlarge the areas of the fluorescent materials. However, simply enlarging the areas easily causes occurrence of problems such as a mixture with the color of the adjacent fluorescent material.
Alternatively, enlarging the area of the fluorescent material corresponding to green, which greatly contributes to the brightness, causes an increase of the electric current for red, which violently changes more than usual, based upon the color temperature. In addition, the focus property may degrade in a low color temperature.